The present invention relates to a steering device in which the ratio of the steering angle to the amount of steering operation can be altered in accordance with vehicle traveling conditions such as the vehicle speed, amount of steering operation and the like.
In the steering device disclosed in Japanese Patent Application Laid-Open No. SHO63-17180, the rotation of the input shaft in accordance with the steering operation is transmitted to the output shaft via a planetary gear mechanism, and the rotation of this output shaft is transmitted to the wheels of the vehicle by a steering gear so that the steering angle varies. The planetary gear mechanism comprises a carrier which holds planetary gears that engage with a sun gear attached to the input shaft, and a ring gear which engages with the planetary gears. The output shaft is attached to this carrier. A sensor which detects the input rotational angle to the input shaft, a sensor which detects the vehicle speed, a motor which rotationally drives the ring gear, and a control device which controls this motor in accordance with the detected input rotational angle and the vehicle speed, are provided. The ratio of the steering angle to the amount of steering operation can be altered in accordance with the vehicle speed by controlling this motor.
Conventionally, the target control amount of the motor has been determined on the basis of the detected input rotational angle and the vehicle speed, and open loop control which controls the motor regardless of the rotational angle of the output shaft has been performed. Accordingly, a stepping motor has been used as the motor.
In cases where steering operation is performed when the vehicle speed is zero, there is an apprehension that sufficient torque cannot be obtained because of using a stepping motor. Furthermore, the stepping motor may lose its synchronism between input and output. Accordingly, the use of, for example, a direct-current motor or the like, which is driven by using the pulse width modulation method in accordance with the target driving current, has been considered as a substitute for a stepping motor. However, in the case of the above described open-loop control, appropriate control cannot be achieved.
The present invention is a steering device in which the rotation of an input shaft in accordance with the steering operation is transmitted to an output shaft via a rotation transmission mechanism, the rotation of the output shaft is transmitted to the wheels of the vehicle by a steering gear so that the steering angle varies, and an electromotive actuator used to drive a constituent element of the rotation transmission mechanism is controlled by a control device in accordance with a variable quantity that expresses a traveling condition of the vehicle, this steering device comprising an input sensor for detecting the amount of input into the input shaft, an output sensor for detecting the amount of output of the output shaft, and a traveling condition sensor for detecting the variable quantity that expresses the traveling condition, wherein the control device controls the electromotive actuator by closed-loop control on the basis of the detection values of the input sensor, output sensor and traveling condition sensor so that the ratio of the transmission of rotation from the input shaft to the output shaft varies in accordance with the variable quantity that expresses the traveling condition.
In the present invention, the electromotive actuator used to drive the constituent element of the rotation transmission mechanism is controlled by closed-loop control, and thus there is no need to use a stepping motor as the actuator.
It is preferable that the rotational angle of the input shaft is detected as the amount of input by the input sensor; the rotational angle of the output shaft is detected as the amount of output by the output sensor; at least one of the vehicle speed and amount of the steering operation is detected as the variable quantity that express the traveling condition; the rotation transmission mechanism is constituted by a planetary gear mechanism in which a planetary gear that engages with a sun gear and a ring gear is held by a carrier, a first planetary gear element that is one of the elements consisting of the sun gear, ring gear and carrier is connected to the input shaft, a second planetary gear element that is another one of the elements consisting of the sun gear, ring gear and carrier and that is not connected to the input shaft is connected to the output shaft, a third planetary gear element that is the other of the elements consisting of the sun gear, ring gear and carrier that is not connected to the input and output shafts is rotationally driven by the electromotive actuator; and the control device stores a first relationship between the amount of input into the input shaft, the variable quantity that expresses the traveling condition and the target control amount of the output shaft, calculates the target control amount of the output shaft on the basis of the stored first relationship, the detection value of the input sensor and the detection value of the traveling condition sensor, stores a second relationship between the target control amount of the electromotive actuator and the deviation between the target control amount of the output shaft and the amount of output of the output shaft, calculates the target control amount of the electromotive actuator on the basis of the stored second relationship, the calculated target control amount of the output shaft and the detection value of the output sensor, and controls the electromotive actuator on the basis of the calculated target control amount.
The amount of input into the input shaft corresponds to the amount of steering operation, and the amount of output of the output shaft corresponds to the steering angle. Therefore, the control for the purpose of altering the ratio of the steering angle to the amount of steering operation can be accurately performed, by directly detecting the amount of input into the input shaft and the amount of output of the output shaft and determining the target control amount of the electromotive actuator on the basis of these detected quantities.
According to the present invention, the ratio of the steering angle to the amount of steering operation can be altered in accordance with the traveling condition of the vehicle without using a stepping motor as the electromotive actuator. Accordingly, there is no insufficiency of torque and loss of synchronism between input and output. Furthermore, since the electromotive actuator is controlled by closed-loop control by directly detecting the amount of output of the output shaft, there is no need to use a servo motor, the output of which corresponds to a target value by detecting the motor rotational angle for the purpose of feedback in the closed-loop control. Specifically, in the case of employing a servo motor, there is a danger that the output of the servo motor cannot corresponds to the target value because it changes while being transmitted to the output shaft via the rotation transmission mechanism. To the contrary, according to the present invention, since the electromotive actuator is controlled by closed-loop control with the amount of output of the output shaft being directly detected and being used as a target value, control for the purpose of altering the ratio of the steering angle to the amount of steering operation can be performed precisely.